OS&Y — outside screw and yoke — is a rising-stem valve construction in which the stem threads are outside the pressure envelope, engaging a yoke nut above the packing. The threads never touch the process fluid, and the exposed stem length shows valve position at a glance. API 600 gate valves are OS&Y by requirement.
Where the operating threads live decides how long a gate valve lasts. In the older inside-screw pattern, the stem threads run in the process fluid — fine for clean water at ambient, fatal for steam, slurry or anything corrosive, which eats the threads and seizes the valve. OS&Y construction lifts the threads clear: the handwheel turns a yoke bushing held in a cast yoke above the stuffing box, the threaded stem rises through it, and only smooth polished stem surface ever passes through the packing.
The benefits compound. Threads run in grease instead of process fluid and can be relubricated in service. The packing seals against plain stem, not threads — a far better sealing surface. And the stem protruding above the handwheel is a mechanical position indicator visible across a pump room, which fire codes (NFPA) exploit by requiring OS&Y gate valves on sprinkler mains precisely so a closed valve is impossible to miss.
API 600 and API 602 gate and globe valves are OS&Y as a matter of standard. All VTXFLOW rising-stem valves use OS&Y construction with renewable yoke bushings.